Generally, separation membranes are made from various inorganic or organic materials, including ceramics, metals and polymers. For example, ceramic structures are oxygen ion conductors and are suitable to cause selective permeation of oxygen ions at high temperatures, such as temperatures of about 500° C. or more. Membranes comprising at least a layer of said ceramic materials are therefore suitable to separate oxygen from oxygen containing gas mixtures.
More specifically, it has been suggested to apply catalyst layers to both sides of a ceramic membrane structure and to connect said catalyst layers externally. On one side of the membrane, the oxygen partial pressure is adjusted to be lower than on the other side of the membrane. In said configuration, oxygen atoms at the side with the higher oxygen partial pressure accept electrons and become oxygen ions, which diffuse through the membrane to the opposite catalyst layer, where they discharge and become oxygen atoms again. The electrons are transferred back via the external circuit to the first catalyst layer. As a result, oxygen is continuously separated from the gas at the side of the membrane which has the higher oxygen partial pressure.
The above-described membranes are also suitable for partial oxidation processes, for instance oxidation of methane gas in order to produce syngas, i.e. a mixture of CO and H2. Syngas is an important intermediate product in the production of methanol, ammonia, or synthetic diesel.
Some oxygen ion conductors also exhibit electron conductivity, referred to as electron-oxygen ion mixed conductors, or just mixed conductors. Alternatively, dual conducting mixtures may be prepared by mixing an ion-conducting material with an electronically conducting material to form a composite, multi-component, non-single phase material.
The following Table lists some of the proposed materials for oxygen separation together with some of their properties.
TABLE 1Properties of membrane candidate materials.σO(S/m), 1073KσO (S/m), 1273K)ρO2(atm)La0.6Sr0.4FeO3-δ1 [1]20 [1]10−17(1273K)10−14(1473K) [2]La0.6Sr0.4CoO0.2Fe0.8O3-δ4 [3]20 [3]10−7(1273K) [2]La0.6Sr0.4CoO3-δ6 [4]40 [4]10−7(1273K) [2]Ba0.5Sr0.5FeO3-δ>4 [5] >8 [5]10−7(1273K) [2]Ba0.5Sr0.5Co0.8Fe0.2O3-δ>27 [5] >47 [5] 10−7(1273K) [2]Ce0.9Gd0.1O1.95-δ6 [6]16 [6]—Ce0.8Gd0.2O1.9-δ6 [6], 20 [7]16 [6], 25 [7]—Y0.16Zr0.84O1.9210—References in Table 1:[1] M. Søgaard, P. V. Hendriksen, M. Mogensen, “Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum ferrite”, J. Solid State Chem 180 (2007) 1489-1503.[2] T. Nakamura, G. Petzow, L. J. Gauckler, “Stability of the perovskite phase LaBO3 (B = V, Cr, Mn, Fe, Co, Ni) in a reducing atmosphere i. experimental results”, Materials Research Bulletin 14 (1979) 649-659.[3] B. Dalslet, M. Søgaard, P. V. Hendriksen, “Determination of oxygen transport properties from flux and driving force measurements using an oxygen pump and an electrolyte probe”, J. Electrochem. Soc., to be published.[4] M. Søgaard, P. V. Hendriksen, M. Mogensen, F. W. Poulsen, E. Skou, “Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum cobaltite”, Solid State Ionics 177 (2006) 3285-3296.[5] Z. Chen, R. Ran, W. Zhou, Z. Shao, S. Liu, “Assessment of Ba0.5Sr0.5Co1−yFeyO3-δ (y = 0.0-1.0) for prospective application as cathode for it-SOFCs or oxygen permeating membrane”, Electrochimica Acta 52 (2007) 7343-7351.[6] S. Wanga, H. Inaba, H. Tagawa, M. Dokiya, T. Hashimoto, “Nonstoichiometry of Ce0.9Gd0.1O1.95−x”, Solid State Ionics 107 (1998) 73-79.[7] N. Sammes, Z. Cai, “Ionic conductivity of ceria/yttria stabilized zirconia electrolyte materials”, Solid State Ionics 100 (1997) 39-44.
Especially flourite and perovskite structured metal oxide materials offer a number of candidates for good oxygen separation membranes. Table 1 lists the oxygen ion conductivity, σo of these materials as well as the pO2 of decomposition at various temperatures (The pO2 of decomposition is estimated as the pO2 of decomposition of LaCoO3 for the Co containing perovskites, and the pO2 of decomposition of LaFeO3 for the Fe containing perovskites). The other listed materials in Table 1 are stable in the pO2 range required for syngas production.
As is evident from the Table, the Co-containing perovskites exhibit a high ionic conductivity. However, they do not posses sufficient thermodynamic stability for operating at low pO2, as is required for instance for production of synthesis gas in a membrane reactor.
On the other hand, of the materials possessing sufficient thermodynamic stability as required for syngas production, doped Ceria possesses the highest ionic conductivity as compared to the above perovskite candidates.
The performance of a mixed conducting membrane will in general be limited by either the electronic or the ionic conductivity, whichever is lower. For the perovskite materials, the ionic conductivity is generally the limiting factor, whereas the electronic conductivity is the limiting factor for the fluorite materials. At high pO2 the performance of Ce0.9Gd0.1O1.95-δ and Ce0.8Gd0.2O1.9-δ will be limited by their electronic conductivity. It has been suggested to enhance the electronic conductivity by using Pr substitution rather than Gd substitution. However, in order to improve the performance of the membrane, for example for the syngas production, new materials are desired exhibiting a better balance of ionic and electronic conductivity to overcome the current limits as provided by the prior art.
Additionally, membranes can be used to separate hydrogen. Hydrogen can serve as a clean fuel for powering many devices ranging from large turbine engines in integrated gasification combined cycle electric power plants, to small fuel cells. Hydrogen can also power automobiles, and large quantities are used in petroleum refining.
In operation, the above described ceramic membranes are exposed to extreme conditions. The opposite sides of the membrane are simultaneously exposed to a highly oxidizing and a highly reducing atmosphere, respectively, at high temperatures. Also the thermal expansion of the membrane at high temperatures might result in stress to the other parts of the apparatus containing said membrane. The membranes therefore need chemical stability with respect to decomposition and should further exhibit low expansion properties.
U.S. Pat. No. 6,139,810 discloses a reactor comprising reaction tubes which comprise an oxygen selective ion transport membrane with an oxidation catalyst side, wherein said membrane is formed from a mixed conductor metal oxide, a heat transfer means formed from metal, and a reforming catalyst disposed about said oxidation catalyst side of said oxygen selective ion transport membrane.
WO-A1-01/09968 relates to mechanically strong, highly electronically conductive porous substrates for solid-state electrochemical devices. A gas separation device is disclosed comprising a first catalyst layer comprising a metal and a second catalyst layer comprising a ceramic material.
U.S. Pat. No. 6,033,632 relates to solid state gas-impermeable, ceramic membranes useful for promotion of oxidation-reduction reactions as well as for oxygen gas separation. The membranes are fabricated from a single-component material which exhibits both, electron conductivity and oxygen-ion conductivity. Said material has a brownmillerite structure with the general formula A2B2O5.
EP-A-0 766 330 discloses a solid multi-component membrane which comprises intimate, gas-impervious, multi-phase mixtures of an electronically-conductive phase and/or gas-impervious “single phase” mixed metal oxides having a perovskite structure and having both electron-conductive and oxygen ion-conductive properties.
U.S. Pat. No. 5,569,633 discloses an ion transport membrane comprising a dense mixed conducting multicomponent metallic oxide layer having a first surface contiguous to a porous layer and a second surface which is coated with a catalyst, wherein the dense mixed conducting multicomponent metallic oxide layer and the porous layer are independently formed from one or a mixture of multicomponent metallic oxides.
U.S. Pat. No. 6,165,553 discloses a method of fabricating a ceramic membrane comprising:                providing a colloidal suspension of a ceramic powder;        providing a polymeric precursor comprising a polymer containing metal cations;        mixing the polymeric precursor together with the colloidal suspension;        applying the mixture to a membrane support to form a composite structure; and        heating the composite structure to form a dense membrane on the membrane support.        
US 2006/0175256 A1 relates to a composite material for purification and filtration of water containing ozone and organic matter which comprises:                (a) a microporous to mesoporous inert ceramic filter, and        (b) a multi-layered, nanocrystalline, sintered ceramic metal oxide catalyst membrane coating on surfaces of the ceramic filter, wherein the catalyst in use degrades the ozone in the water into a hydroxyl or other radical in situ which reacts with the organic matter by the composite ceramic membrane during filtration.        
However, the membranes proposed in the prior art do not result in membranes having a good balance of ionic and electronic conductivity, limiting the membrane efficiency due to the inherent limit of either the electrical or ionic conductivity of the employed materials. On the other hand, the suggested materials showing a promising balance are chemically unstable structures not being suitable for membrane mass production, as the membranes have a very short life time. There is thus still a need for membrane structures which are cheap, provide a good balance of a mixed ionic and electronic conductivity while exhibiting a chemical stability under the relevant oxygen partial pressures.